The purification of materials is an important step in many commercial and industrial processes. The achievement of economical removal of impurities from a material, thereby increasing its purity, is a major goal in the optimization of these processes. However, efficient methods of separating impurities from a desired material, especially on a large-scale, are often both elusive and difficult to employ.
Crystallization of materials is one method used to remove undesired impurities. In a crystallization, a material with impurities is dissolved in a solvent and then caused to crystallize back out of the solution, forming a more pure form of the material. While crystallization can be an economical manner of purification, some materials do not cleanly crystallize out of a solvent, but rather crystallize first as the relatively pure desired material, and then upon those crystals a combination of the desired material and the impurities form. Sometimes this effect can be accentuated in situations where the yield of the crystalline desired product from the solution is attempted to be maximized. In other cases, inherent properties of the system of the solvent and the dissolved materials are such that cleanly stopping the crystallization before the undesired materials are deposited on the pure crystals is difficult or not possible.
Another problem that can sometimes occur in using crystallization to purify a material is difficulty in separating pure crystals that have formed in the solution from the surrounding solution. In an ideal crystallization, as crystals grow out of a solution, the impurities are left behind in the solution, and the final step of purification is removal of the solution containing the impurities from the crystals. If the solvent is not entirely removed from the crystals, impurities can be left behind on the surface of the crystals. In cases where the solvent is a molten metal, solid solvent can be left behind on the surface of the crystals.
For example, when molten aluminum is used to crystallize silicon, the silicon flakes (crystals) are coated with an aluminum-silicon alloy after draining the mother liquor away from the flakes. To use these flakes for making further purified silicon, or to use these flakes directly, the aluminum-silicon alloy needs to be removed from the surface of the flakes and reduced as much as possible in the flakes. Crystallization is not in itself economically sufficient to bring the flakes to the desired level of purity, an additional means of purification is needed.
Therefore, there is a need for efficient large-scale purification methods of materials.